The present invention relates generally to monitoring the dry density of a board product made by a wet forming process and more particularly to monitoring its dry density in-line, that is, without removing a sample, and preferably continuously throughout the manufacture of the board product.
One common way in which board products such as perlite insulation board or mineral wool acoustical board are made is by a wet forming process. In accordance with this process, a dilute aqueous slurry comprising at least some of the ingredients or components of the product is prepared. This slurry is deposited on a moving screen, either a generally horizontal screen comprising part of a Fourdrinier type machine or a cylindrical screen comprising part of an Oliver machine, to form a moisture-laden sheet thereon. This moisture-laden sheet moves between a number of press rolls and ultimately into a dryer where its moisture is removed so as to form the final board product.
It should be obvious that there are a number of physical characteristics which affect the ultimately formed board, from not only a performance standpoint but also from a cost standpoint. For example, if the ultimate product is formed thicker than the specifications require, the product may be more expensive than need be, whereas if the ultimate product is too thin its performance may be seriously compromised. In a similar fashion, it may be quite costly to make a product which is more dense than required, and where insulation values or acoustical values are critical an overly dense product may seriously compromise these values. Still another factor to be considered and one which will not necessarily affect the ultimate product but rather its process of manufacture is the moisture content in the moisture-laden sheet. It can be quite costly from an energy usage standpoint to include more moisture in the sheet than is necessary. By the same token, where the sheet is not formed with sufficient moisture the ultimate physical characteristics of the end product could be adversely affected.
From the foregoing, the importance of monitoring the dry density and thickness of a board product of the type to which the present invention is directed and the monitoring of its moisture content when still in a moisture-laden state should be quite clear. However, Applicants have found that the data obtained for a given sample in this monitoring procedure, for most efficient use of the procedure, must be obtained as soon as possible after initial formation of the sample. More specifically, when this data is obtained a prolonged period after formation of the sample, the data is not nearly as helpful in making adjustments to the overall process as would be the case if the data were obtained immediately after formation of the sample. And yet, to Applicants' knowledge, the typical monitoring system being used today requires a substantial time lag between formation of a given sample and obtaining data relating to its dry density.
In accordance with this typical monitoring procedure, the moisture-laden sheet on the moving screen is monitored for wet weight and thickness, this being accomplished continuously and in-line throughout the manufacturing process. However, its moisture content is not obtained in this way and hence neither is its ultimate dry density. Rather, to obtain moisture content, a given sample is actually removed from the line, weighed, and placed in an oven to dry. It is then weighed again and from this dry weight along with the wet weight and thickness of the sample the dry density of the sample is calculated. However, this drying procedure takes at least 20 minutes (generally longer) and hence by the time the data is obtained the process may be running entirely different than at the time the sample was measured. This of course makes it difficult if not completely unreliable to make adjustments in the process based on the dry density obtained. Specifically, the operator is never certain that the hand samples made at least 20 minutes earlier are representative of the wet weight, thickness and moisture content of the moisture-laden sheet presently being formed on the moving wire or that it is representative of the dry density of the product being formed from the present moisture-laden sheet. Hence, it is difficult, if not impossible, for the operator to accurately and reliably make the appropriate changes to the process where necessary.
As will be seen hereinafter, the monitoring procedure of the present invention eliminates the long time delay between formation of the sample to be monitored and the actual monitoring thereof. In this way, any corrections in the overall process to compensate for undesirable physical characteristics can be made immediately. Moreover, by monitoring any given sample and obtaining data on that sample immediately after it is formed and preferably by doing this continuously in-line the operator obtains immediate feedback as to the success of his process adjustments and hence can quickly learn what process manipulations are required to continuously maintain the ultimate product within specification.